Ideal wall insulation b/w living/garage


Old 03-04-10, 10:01 AM
Thread Starter
Join Date: Jan 2010
Location: PA
Posts: 10
Ideal wall insulation b/w living/garage

Hi folks,

Our project continues, we now have this question: I have a new 2x4 wall separating garage from living space. We are soon ready for insulation and drywall and I am looking for ideal way to insulate/finish. My plan, from the interior out to garage side is:

drywall, plastic vapor, batt insulation, fire-rated drywall

Would others add anything b/w fire-rated and insulation?

Any other thoughts?

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Old 03-04-10, 03:33 PM
Join Date: Aug 2009
Location: USA
Posts: 4,941
Like what? Another vapor barrier would be bad news. If you want more insulation you could add rigid foam over the 2x4's but make sure that foam is completely covered by drywall for fire protection. If you want more fire protection, you could add another layer of drywall. Using mineral wool batts instead of fiberglass may also help in that respect.
Old 03-08-10, 06:26 AM
Join Date: Jan 2006
Posts: 143
Need R-19 For the Wall

Unless you will have an insulated and heated garage, your plan does not meet Code in Pennsylvania.


Because the wall between the garage and living space is part of the thermal envelope and needs to be insulated to a minumum of R-19.

You cannot get R-19 in a 2x4 wall unless you use compressed R-13 batts in the cavity and place a 1" thick minimum R-5 continuous foam insulation on the garage side of the studs under the garage side drywall.

(Pennsylvania Energy Code will allow this 13+5 insulation combination)

So from the garage side moving in, you will need:

-1/2" minimum drywall (Not required to be fire-rated)
-1" foam insulation
-2x4 studs with R-13 compressed fiberglass
-1/2" interior side drywall

Whether or not you will need a vapor retarder on the fiberglass batts will depend upon whether you use a closed cell or open cell rigid foam.

Closed cell foam: no vapor retarder over fiberglass...
Open cell foam: faced fiberglass required
Old 03-08-10, 06:44 AM
Join Date: Oct 2008
Location: New England
Posts: 9,995
Hi Joe, just to add.
manhattan42, I'm not sure what you are referring to as compressed R-13 as that should be the normal thickness for a 2x4 wall. They do make a high density R-15 or as drooplug suggested the mineral wool batts are a bit higher in r-value. The mineral wool is also a good extra barrier for fire protection.

I would add the importance of air sealing from foundation up as the garage can be a source of exhaust and other fumes.

Do confirm your code requirements as manhattan42 stated. My only question on his recommendations would be the closed cell rigid on the garage side to act as a vapor barrier. To use a rigid foam as a Vb on the outside in cold country, there is a balance point that keeps the dew point out of the wall cavity and I believe it would require more than 1 inch of rigid. Simply using 1" pink or blue with its higher permeability rating and no plastic or foil covering could be OK, but has to also be OK with the code people. Right or wrong, they rule.

Old 03-08-10, 07:34 AM
Join Date: Jan 2006
Posts: 143

R-13 IS 'compressed' fiberglass.

It is the 'high-density' form of R-11 and designed to fit in 2x4 cavities...

It is unlikley that vapor will ever condense on the warm-in-winter side of the wall cavities when using R-5 foam on their exterior regardless of open or closed cell type...even in extremely northern climates...and why Energy Codes all around the US permit foam installation in the manner prescribed and as I mentioned above.

Since 'closed' cell foam IS a vapor retarder while 'open' cell foam is not....One never wants to install a vapor retarder on the warm-in-winter sides of a wall when closed cell foam is used on the exterior....otherwise you have a 'double vapor barrier' problem on your hands.
Old 03-08-10, 07:56 AM
Join Date: Oct 2008
Location: New England
Posts: 9,995
Just so we give the op the correct information, R-13 is a medium density insulation for 2x4 walls. The batts are also available in high density, R-15.
Energy Savers: Fiberglass Insulation Material

And for the balancing act with vapor barriers, it is much better to not place a VB on the outside, regardless of what is on the inside. Given that this is a garage, it will not see the deep cold weather that the outside of the house will see, but R-5 rigid on the outside WILL still allow the dew point to fall inside the wall cavity. Thus VB on the inside and if rigid is used on the outside, it should be permeable enough to allow drying to the outside.

Newer research is actually saying to omit the VB in many cases as air sealing is what prevents most moisture movement and the ability to dry is more important. Codes still must be followed, and you may be correct in what PA requires/allows, I'm a few hundred miles away.

No flame intended
Old 03-08-10, 08:47 AM
Join Date: Jan 2006
Posts: 143

Thanks for noting some of the fine points regarding 'medium density' vs 'high density' fiberglass...and your opinions regarding vapor retarders and foam insulations... but these are not really relevant to the main point of this discussion.

The main point of this discussion is that the original poster's design violates energy code requirements in Pennsylvania because it does not meet minimum wall insulation thicknesses.

The 2006 International Energy Conservation Code under which the poster is required to build in Pennsylvania prescriptively requires either a minimum of R-19 wall cavity insulation (2x6 framing) or R-13 cavity insulation with a continuous insulation on the exterior of a 2x4 wall assembly of R-5.

Alternative designs are permitted where approved by the local code official....but unless the original poster has an approved alternate design (such as RESCheck) that might permit less than R-19 in the walls...his design remains flawed.

It should be noted that the International Energy Conservation Code is developed in large part with research conducted through the US Department of Energy and private research organizations such as the Building Sciences group that so many like to cite.

That said, the prescriptive elements in the IECC have already taken into account things like 'dew points' when foam insulation is permitted on the exterior of a building...and have approved them for use in locations so noted.

These locations include all the New England States as well as Pennsylvania...

So without dragging this conversation off course any further, let it simply be said that the original poster's design is not acceptable for Pennsylvania under any circumstance, and his only alternative at this point is to add the foam to the garage side wall....

....And the picking at nits about vapor barriers, dew points, open/closed cell foams, 'exceptionally tight construction', or 'high' vs 'medium' density fiberglass batts left for another topic on another day.


Pennsylvania Certified Energy Code Inspector
Old 03-08-10, 10:50 AM
Thread Starter
Join Date: Jan 2010
Location: PA
Posts: 10
Excellent Info

Thank you Droop, Bud and Manhatten:

Now that's what I call some helpful input!

Based on everything I just read I will plan to add the foamboard insulation to the garage side, prior to drywall. Seems like the vapor barrier is less important - I'd rather make sure I have the higher R value.

Thanks again.


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